Acidification
Background
It was estimated in 2002 that some 17% (62kms of main stem and tributaries) of the Upper Wye and Irfon system was suffering from the effects of acidification. Fortunately the Usk catchment lies outside the affected geology. For comparison, other welsh rivers affected include the Tywi (39km), Conwy (8.4km) and Glaslyn (5km).
Acidification, the result of acid rain, adversely affects all organisms in a stream from diatoms to fish. In Wales these streams are characterised by having extremely low pH events following rainfall. Further investigation shows impoverished invertebrate life and invariably no fish. In appearance, these streams appear to be 'crystal' clear and just touching a stone will lead you to note that it is often devoid of any slime.
This problem occurs where high rainfall combines with base poor soils and strata and it is exacerbated by commercial forestry. Fortunately, the habitats of these streams remain largely undamaged. In the Wye, the large early running springers spawned in these streams and so there is ample incentive to find an effective treatment.
The Rationale
Several projects to correct the problem have met with varying degrees of success including mechanical dosing schemes on the Twyi and Hafren. The former project was carried out above the impoundment of Lyn Brianne and so it was not necessary to have the smaller order streams improved for migratory fish spawning. However, these streams are crucial to the requirements on the Wye.
Professor Steve Ormerod and colleagues published a long-term survey of work previously carried out in the 80s. When acidification first appeared, he conducted a series of controlled experiments to determine an effective way of curing the problem. A review of this work in 2002 demonstrated that liming the actual hydrological source of a stream (the uppermost part from which streams arises) not only provided the right neutralising effect, but also that the benefit lasts for more than a decade. Additionally, the 'cure' was effective over the whole stream.
'Hydrological Source Liming', as this technique was termed, had been carried out using helicopters to deliver the lime and was thought to be too expensive and possibly too invasive.
Work to correct the effects of acid rain
Success would depend on developing an effective and cheap delivery system that we could manage ourselves, and on persuading statutory bodies that the benefits far outweighed any downsides. We also needed to apply a monitoring programme that could determine unequivocally whether and to what extent we had been successful. The Foundation is extremely grateful to Profs Steve Ormerod and Brian Reynolds, Dr Ingrid Juettner and their colleagues for devising, setting up and evaluating the monitoring of this work and for calculating the required dosing rates.
Similarly, we were fortunate that some key local owners, the Forestry Commission and our partners were far sighted enough to support the project. Thanks are also due to Tarmac Western in Radnorshire who supplied the correct grade of lime and the vital lightweight machine to deliver it.
Work started in 2003 and we learnt very quickly how difficult liming would be in these remote areas. 2004 was washed out but 2005 2006 and 2007 were ideal years for lime distribution. New skills were learned. By 2007, the required dose had been reached on the upper Wye but not on the Irfon. A significant problem was that many of the hydrological sources had been obliterated by the forestry drainage schemes, leaving no available sites in which to deliver the lime.
To counter this we tried 'sand liming' on a single severely acid tributary - site 56 - in 2006 and subsequent monitoring suggested a significant water quality improvement. Sand liming involves applying larger (sand sized) chippings of calcium carbonate to first and second order streams. While forestry deprived us of hydrological sources, their intricate road system made this a relatively easy task. Streams without adequate sources have now received sand lime.
Results so far
Monitoring consisted of the investigation of the following:
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Diatom analysis
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Invertebrate analysis
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pH, conductivity and water chemistry
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Fish densities
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Distribution of fish
The map below shows the monitoring sites and areas limed on the Upper Wye. Please click on the dates to the left of the map for a historical comparison.
1975 Survey conducted as part of Craig Goch Study. Please note the extent to which salmon fry extend to the extreme upper Wye. However, the Tarennig had no fish, even then.
